yeah, in the stylesheet use the png as the default backgroung image for wherever you want it.
Then, make use of the IE6 hack, and tell it to make the defauly image as the gif.
i.e.
#header{
background:url(images/image.png) 0 0 no-repeat;
}
*html #header{
background:url(images/image.gif) 0 0 no-repeat;
}
Now, in any browser except for IE6, the background will be the png, and in IE6 the background will be the gif.
good luck?